A heat-sealing film for sealing a container made of e.g. plastic or paper, is used also as a cover film which is a cover material for packaging an electronic component, as represented, for example, by a carrier tape.
Such a heat-sealing film may be one having a two layer structure comprising a stretched film to maintain tear strength and break strength and to provide heat resistance for heat-sealing and a heat-sealing layer to present a fusion bonding property by heating. However, one having a three layer structure or a higher multilayer structure having an interlayer disposed between the stretched film and the heat-sealing layer, to provide an improvement of the mechanical strength, etc., is widely used. Such a cover film having a three layer or higher multilayer structure is produced by an extrusion-laminating method by utilizing the heat-sealing property of the heat-sealing layer or the interlayer interposed between the heat-sealing layer and the stretched film. However, by this method, the number of extrusion-laminating steps increases as the number of layers increases, whereby there will be a problem such that the productivity deteriorates, or the raw fabric loss increases thereby to increase the cost. Also from the aspect of the quality of the product, the possibility of inclusion of foreign matters increases as the number of steps increases.
The heat-sealing film is required to have the following properties as the basic properties:
(1) a heat-sealing property to readily obtain is practical peel strength, and
(2) a readily openable property so that at the time of opening, the content can easily be taken out without scattering. In recent years, an improvement is desired also with respect to (3) transparency. If the transparency is good, the packaged content can easily be ascertained, whereby the inspection operation may be facilitated, the reliability may be improved, and reassurance may be obtained.
For example, JP-B-57-53828 or JP-B-57-42652 discloses a heat-sealing film which is excellent in the heat-sealing property and which is readily openable. However, it does not necessarily fully satisfy the requirement for transparency. Accordingly, a heat-sealing film having better transparency is required.